1. Field of the Invention
The present invention generally relates to an optical device for optically connecting a pair of optical fibers (a first and a second optical fiber) and, more particularly, relates to an optical device having two or more functions including the function of an optical isolator and suitable for use in such an optical system as an optical amplification system and a two-way optical transmission system.
2. Description of the Related Art
As references disclosing optical devices for connecting a first optical fiber with a second optical fiber, specifications of U.S. Pat. No. 4,548,478 and Japanese Patent Publication No. 61-58809 are known. The optical devices disclosed in such specifications have the function of an optical isolator, of which the forward transmittance is not dependent on the polarization state of the input light beam (polarization-independent isolator). The polarization-independent optical isolator in principle couples all of the light rays output from a first optical fiber to a second optical fiber and couples none of the light rays output from the second optical fiber to the first optical fiber. Hereinafter, the polarization-independent isolator will be briefly referred to as "optical isolator".
The optical isolator of the described type has many uses. For example, in an optical amplification system provided with an optical amplification medium of the optical pumping type, there is used an optical isolator to prevent formation of an optical resonator structure including the optical amplification medium in the resonating optical path. As a practicable optical amplification system including an optical amplification medium of the optical pumping type, there is known an optical fiber amplifier including, as the optical amplification medium, an optical fiber (doped fiber) at least the core of which is doped with a rare earth element such as Er, Nd, and Yb.
In the optical fiber amplifier, signal light and pumping light are introduced into the doped fiber and the signal light is directly amplified on the principle of stimulated emission at the wavelength of the signal light. Therefore, in order to construct an optical fiber amplifier, a multiplexer for introducing the signal light and the pumping light supplied through separate routes into a single doped fiber and other peripheral parts become indispensable. If an optical device having functions of two or more of such peripheral parts is realized, it will be very useful in simplifying the structure of an optical amplification system or the like.
Accordingly, an object of the present invention is to provide an optical device being simple in structure and having two or more functions including the function of an optical isolator.